There are various standard analog color television (TV) broadcast systems, including the National Television System Committee (NTSC) system used in the United States and Japan, the Phase Alternation by Line (PAL) system used mainly in western Europe, and the Sequential Coleur avec Memoire (SECAM) system used in France and elsewhere. Due to the spread of video tape recorders (VTRs) and video games, non-standard video signals other than the signals (standard signals) used in the above standard television broadcast systems can also be found. In recent years, video signal processing devices that perform digital signal processing of standard and non-standard video signals used in several different types of television broadcast systems have been developed.
In such digital video signal processing, the analog video signal is converted to a digital signal (A/D conversion) by use of a prescribed sampling clock, and then converted from a composite signal to a luminance signal (Y signal) and a chrominance signal (C signal) by a process referred to as Y/C separation.
Conventional video signal processing circuits perform Y/C separation by generating a burst locked clock based on the color subcarrier frequency (fsc) of a burst signal imposed on the blanking interval of the composite signal (as a reference signal for the chrominance signal phase and amplitude; see, for example, Patent Document No. 1).
To support multiple television broadcast systems, another type of video signal processing circuit performs Y/C separation by converting a composite video signal that has been sampled on a single common free run clock to sampling data with a frequency four times that of the burst locked color subcarrier frequency (hereinafter, 4fsc; see, for example, Patent Documents No. 2 and No. 3).
A general method of Y/C separation uses a horizontal frequency separation filter based on the frequency band of the chrominance signal (hereinafter, one-dimensional Y/C separation). In the NTSC system, two-dimensional Y/C separation and three-dimensional Y/C separation can provide higher picture quality: two-dimensional separation uses a line comb filter, exploiting the fact that the phase of the color subcarrier is inverted on alternate horizontal scanning lines (see, for example, Patent Document No. 4); three-dimensional Y/C separation uses a frame comb filter, exploiting the fact that the color subcarrier phase of the same horizontal scanning line is inverted in alternate frames (see, for example, Patent Document No. 5).
Two-dimensional or three-dimensional Y/C separation is based on the correlation between lines or frames (line-to-line or frame-to-frame color subcarrier phase relationship). The Y/C separation process uses, for example, the property of the standard NTSC signal that the color subcarrier phase inverts (the phase changes by 180°) at the horizontal period (the line-to-line period) or the frame period.
In a number of television broadcast systems the burst locked line-to-line or frame-to-frame phase relationship differs from the relationship in the NTSC system: in the NTSC-4.43 system and PAL system, for example, the phase of the video signal does not invert; in the non-standard signals used in VTRs and video games, the line-to-line phase relationship decays and the phase does not always invert by 180°. If the line-to-line difference in the color subcarrier phase is not exactly 180°, two-dimensional or three-dimensional Y/C separation cannot separate the luminance and chrominance signals accurately. The reduced accuracy results in dot crawl and other picture quality problems.
Since two-dimensional Y/C separation or three-dimensional Y/C separation cannot always be applied to the composite signals of those television broadcast systems that lack the above color subcarrier phase relationship, or to non-standard signals in which the color subcarrier phase becomes displaced, one-dimensional Y/C separation or two-dimensional Y/C separation is performed selectively, according to the standard or non-standard signal of each television broadcast system (see, for example, Patent Documents No. 3. No. 6, and No. 7).
Patent Document No. 1: Japanese Patent Application Publication No. H10-164618 (FIG. 1)
Patent Document No. 2: Japanese Patent Application Publication No. 2001-112016 (FIG. 1)
Patent Document No. 3: Japanese Patent Application Publication No. 2002-315018 (FIGS. 1 and 6)
Patent Document No. 4: Japanese Patent No. 2566342 (FIG. 1)
Patent Document No. 5: Japanese Patent Application Publication No. H1-174088 (FIG. 1)
Patent Document No. 6: Japanese Patent Application Publication No. H7-131819 (FIG. 1)
Patent Document No. 7: Japanese Patent Application Publication No. 2003-92766 (FIG. 1)